Within the home and workplace, Ground Fault Circuit Interrupters (GFCI's) designed to interrupt power provided to various loads, such as household appliances, consumer electrical products and branch circuits are in high demand. Specifically, electrical housing code regulations in most states require that new housing construction have bathrooms and kitchens equipped with Ground Fault Circuit Interrupters (GFCI). Circuit interrupting devices, such as the GFCI device described in commonly owned U.S. Pat. No. 4,595,894 and incorporated herein in its entirety by reference, use a trip mechanism to mechanically break an electrical connection between an input and one or more output conductors, when, for example, a ground fault is sensed by a sensing circuit incorporated within the circuit interrupting device.
The sensing circuit typically incorporates a high turn ratio differential transformer connected to detect the difference of current in the neutral and phase wires of an electrical distribution system. The secondary of the differential transformer is connected to an input of a high gain operational amplifier (OA) on a chip having an integrated circuit for detecting ground faults. The high gain operational amplifier provides an amplified signal which is proportional to the differential of line and phase current and this signal is used to control a silicon controlled rectifier (SCR).
Ideally, under a ground fault condition, the amplified signal will exceed a certain predetermined reference level and enable a triggering device such as the SCR to effect the operation of a relay coil assembly, such that the contacts of the relay coil assembly will open to disconnect the load from the line. Noise, radio signals, RF producing equipment such as cell phones, and other phenomena (either radiated in free space or conducted within the electrical distribution system), can affect the operation of GFCI circuits. In particular, electromagnetic interference (EMI) having radio frequency interference (RFI) signals which are within a specific radio frequency band may effect the operation of the GFCI circuit. This can occur when spurious energy is coupled to the inputs of the high gain operational amplifier. This coupling can begin with the current carrying conductors in the electrical distribution system itself. The conductors can either carry conducted noise from a source connected to some other branch of the system, or they can act as antennas, or receptors, to convert radiated signals (such as radio waves) into conducted signals within the electrical distribution system. These spurious signals may then be coupled from the line conductors to the secondary winding of the differential transformer.
Once the spurious signals are coupled into the secondary winding of the transformer, they are conducted through circuitry to the inputs of the operational amplifier where a component of the signal can influence the operation of the amplifier in such a way to either increase or decrease the functional ground-fault sensitivity which may create nuisance or false tripping of a GFCI.
More particularly, an AC coupling capacitor, typically connected at the input of the operational amplifier, charges up when a transient signal appears on the differential transformer. When the impedance in the discharge path of this AC coupling capacitor is low, the AC coupling capacitor will discharge rapidly. Conventionally, however, the only impedance in the discharge path that exists is the inherent impedance located at the input of the operational amplifier which is normally very high. Thus, the AC coupling capacitor will discharge slowly and a false signal that is received at the operational amplifier may trip the GFCI.
Alternatively, nuisance tripping under no-load, no fault conditions may exist in extreme cases. As a result, this increased ground-fault sensitivity may increase the ground-fault threshold trip to above 6 milliamps, which is the present industry standard for the upper limit for ground fault threshold. Another result is that the GFCI device will not trip in the presence of a 2 ohm grounded neutral loop, which is the standard for the highest loop impedance for tripping. In extreme cases, it is theoretically possible that the GFCI will be rendered inoperable.
Other critical components in a GFCI circuit that are sensitive to RFI include the silicon controlled rectifier (SCR) and the power supply. Thus, a need exists for a circuit interruption device that has improved immunity to noise interference such as EMI and RFI interference.
The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.